The crowd, gathered in freezing temperatures in central Brussels, chanted “Free Olivier” and sang Happy Birthday to mark Vandecasteele’s 42nd birthday this week.
“The aim is to show the government that we cannot leave an innocent person there,” his sister Nathalie told AFP.
Olivier Van Steirtegem, a spokesman for the family, insisted that “every minute, every second counts” and that they hoped growing public pressure for his release would push the Belgian authorities to find a solution faster.
Iran arrested Vandecasteele in February 2022, and he has since been held in conditions that Belgium’s government has described as “inhumane”.
The authorities sentenced him this month to more than 12 years behind bars for “espionage” as well ordering him to be subjected to 74 lashes.
UN rights experts have slammed Vandecasteele’s detention as a “flagrant violation” of international law.
His backers and rights groups contend he is being held as part of Iran’s “hostage diplomacy” to try to get Belgium to release an Iranian diplomat incarcerated for terrorism.
The diplomat, Assadollah Assadi, was found in guilty in 2021 of masterminding a plot to blow up an event organised by an Iranian exiled opposition group outside Paris in 2018.
The plot was foiled by European intelligence services, and Assadi, a diplomat stationed in Austria who was identified as having provided the explosives for the bomb, was sentenced to 20 years in prison.
In July last year, Belgium and Iran signed a prisoner-swap treaty that Brussels viewed as a path to free Vandecasteele.
But Belgium’s Constitutional Court suspended the treaty after exiled Iranian opposition members challenged it on the grounds it would lead to the release of Assadi.
The constitutional court said the suspension was in place pending a ruling on the legality of the treaty.
The Belgian government insists it is doing “all it can” to have Vandecasteele released and says it has strengthened its legal team to plead the case.